This invention relates to improvements in a method of forging a raw material for sintering and forging in order to produce a forging to be used as a mechanical part or the like, and more particularly to the method of forging a sintered compact containing iron as a main component and graphite.
Hitherto forging has been widely used for producing mechanical parts. Additionally, in recent years, it has been studied to produce a mechanical part first by sintering compacted metallic powder to form a sintered compact and then by forging the sintered compact. The metallic powder contains iron as a main component and further contains a certain amount of graphite. It has been known that crack tends to be readily produced in a product by making extrusion forging on such a sintered compact.
This fact is described, for example, at pages 38 and 39 of a technical text xe2x80x9cIndustrial Library 13xe2x80x94High Speed Forging (published on Jun. 25, 1969 by Nikkan Kogyou Shinbunsha)xe2x80x9d. According to this technical text, iron powder is subjected to pre-compacting and sintering thereby to form a sintered compact having a relative density of 78%, and then the sintered compact undergoes extrusion forging under pressing upon loading a back pressure of 4000 kg/cm2. This technical text recites that production of crack cannot be prevented. Additionally, the technical text recites that production of crack can be prevented in case that the above sintered compact is subjected to extrusion forging with a high speed hammer loading a back pressure of 3000 kg/cm2.
In the latter forging method, production of crack can be prevented; however, a forming speed during the forging is high to generate heat thereby inviting another disadvantage that such heat causes the dimensional accuracy of a forging to lower.
Apart from the above, in recent years, a forging method as disclosed in Japanese Patent Provisional Publication No. 2000-17307 has been devised and proposed. This forging method is summarized as follows: Metallic powder is compacted to form a compact having a certain density. Thereafter, the compact is sintered at 1300xc2x0 C. under vacuum thereby forming a sintered compact. The sintered compact is located in a die and pressurized from upward and downward directions under heating, in which a pressure from the downward direction is reduced relative to that from the upward direction thereby accomplishing extrusion forging. According to this forging method, production of crack in a forging can be prevented under the effects of heating during the extension forging and application of the pressures from upward and downward directions.
However, drawbacks have been encountered in such a conventional forging method. Specifically, in case that metallic powder as a raw material is prepared by mixing graphite with metal powder containing iron as a main component, graphite is excessively diffused in the metal powder to largely increase the hardness of the sintered compact. Accordingly, if sufficient heat is not applied to the sintered compact during the succeeding extrusion forging, production of crack will occur in the resultant forging. Thus, in the conventional forging method, carrying out such high temperature heating is required during the extrusion forging, thereby large-sizing and complicating a facility or forging machine upon addition of a heating device while shortening the life of the die and lowering the dimensional accuracy of the resultant forging.
In view of the above, it is an object of the present invention to provide an improved method of forging a raw material for sintering and forging, which can effectively overcome drawbacks encountered in conventional forging methods.
Another object of the present invention is to provide an improved method of forging a raw material for sintering and forging, which can securely prevent production of defects such as crack and the like of a resultant forging without inviting large-sizing and complication of a forging facility or machine, shortening the life of a die and lowering the dimensional accuracy of the resultant forging.
An aspect of the present invention resides in a method of forging a raw material for sintering and forging. The method comprises the steps of: (a) compacting metallic powder containing iron as a main component and graphite to obtain a compact having a predetermined density; (b) sintering the compact at a temperature ranging from 700 to 1000xc2x0 C. to form a sintered compact having a texture in which graphite is retained at grain boundary of metal powder; (c) compressing the sintered compact from two directions to obtain a compressed sintered compact; and (d) extruding the compressed sintered compact upon pressing the compressed sintered compact from the two directions in a manner that a pressure in one of the two directions is reduced relative to a pressure in the other of the two directions to accomplish extrusion forging. Preferably, metallic powder contains at least one selected from the group consisting of as chromium, molybdenum, manganese, nickel, copper, tungsten, vanadium and cobalt.
Another aspect of the present invention resides in a method of forging a raw material for sintering and forging. The method comprises the steps of: (a) compacting metallic powder containing iron as a main component and graphite to obtain a compact; (b) sintering the compact at a temperature ranging from 700 to 1000xc2x0 C. to form a sintered compact having a texture in which graphite is retained at grain boundary of metal powder; (c) filling the compact in a forming space of a die; (d) compressing the sintered compact in the forming space of the die from opposite directions without heating to obtain a compressed sintered compact; and (e) extruding the compressed sintered compact in the die without heating by controlling pressures in the opposite directions in a manner that the pressure in one of the opposite directions is decreased relative to the pressure in the other of the opposite directions to accomplish extrusion forging.
According to the present invention, in the sintered compact obtained by sintering the compact at 700 to 1000xc2x0 C., binding among metals progresses in such a manner as to be able to make a compression deformation while graphite is hardly diffused and is dispersed at grain boundary. When this sintered compact is compressed from two directions, it can be easily compression-deformed under cold compression thereby forming the high density compressed sintered compact. Then, this compressed sintered compact is compressed from the two directions, in which the pressure from one direction is reduced relative to that from the other direction. As a result, the compressed sintered compact is cold-extruded from the side of the other direction thereby obtaining a forging having no defects such as crack and the like.
Preferably, the predetermined density of the compact is not lower than 7.1 g/cm3. With this feature, metal powder is in a condition where contact among metal particles of the metal powder is increased. Additionally, the composition of the sintered compact is in a condition where graphite is retained at grain boundary of the metal powder while precipitates such as carbide and the like are hardly formed. As a result, the sintered compact is high in hardness and high in elongation percentage while lubricating characteristics at grain boundary of metal powder is increased thereby to wholly raise the deformability of the sintered compact. These effects are combined with the above effects of the particular forging process thereby making it possible to prevent production of defects such as crack and the like.
Preferably, the compressing step and the extruding step are successively carried out. With this feature, the sintered compact which has been subjected to a forming process at the compression step can be transferred to the succeeding extruding step without its work hardening. Accordingly, extrusion forging can be made without trouble even a raw material which tends to readily make its work hardening.
Preferably, the compressing step and the extruding step are carried out without heating the sintered compact. With this feature, the dimensional accuracy of the resultant forging can be raised while thermal deterioration of a die can be prevented.
Preferably, the sintered compact is extruded under a forward extrusion in the extruding step. With this feature, forging of a long member can be realized without inviting crack or the like of the long member.
Preferably, the step of preparing a die which has a compression section formed with a first space in which the sintered compact is set to be compressed, and an extrusion section continuous with the compression section and formed with a second space continuous with the first space of the compression section. The second space is smaller in sectional area than the first space. Here, the compression step is carried out by the compression section to increase a density of the sintered compact to form a compressed sintered compact which is to be extruded into the extrusion section, and the extruding step is carried out by the extrusion section successively to the compression step to form a forging. With this feature, the compression section and the extrusion section are formed continuous in the die, so that the compression step and the extrusion step are successively carried out.
Preferably, the first space of the compression section of the die is shaped corresponding to a final product or resultant forging. With this feature, a further processing is unnecessary onto a part of the material remaining in a not-extruded state in the compression section of the die, and therefore the material in the compression section can be used as a product as it is.